1. Field of the Invention
The present invention relates to a tracking apparatus which is applied to an imaging device such as a digital camera and which improves the performance of tracking a tracking target subject.
2. Description of the Related Art
Techniques for tracking a subject are known in, for example, Jpn. Pat. Appln. KOKAI Publication No. 2007-011054 and Jpn. Pat. Appln. KOKAI Publication No. 2009-268086. Jpn. Pat. Appln. KOKAI Publication No. 2007-011054 discloses the use of, for example, an AF evaluation value used in contrast autofocus (contrast AF) to track a subject. According to the disclosure in this publication, an autofocus lens is driven, and at the same time, a contrast detection signal are calculated regarding a plurality of AF areas in accordance with an output signal of an image pickup device. The movement of the subject between the AF areas is detected from the calculated contrast detection signal. An in-focus position is found in accordance with the detection result.
Jpn. Pat. Appln. KOKAI Publication No. 2009-268086 discloses tracking a subject by using at least two regions for tracking the subject. According to the disclosure in this publication, a first region and a second region for tracking the subject are provided on image data acquired by imaging the subject. The first region is always used as a standard region to track the subject. The second region is used independently of the first region to track the subject.
The tracking process is briefly described for reference. FIG. 21 shows an example of a frame image for a given timing in tracking operation. FIG. 22 shows an example of a frame image taken one frame later in the tracking operation. In an imaging screen 1 of a digital camera, a subject (subject to be tracked) A such as a person to be tracked exists, and a tracking frame W for tracking the subject A is displayed at a tracking position B1. A plurality of two-dimensionally arranged focus detection areas C are shown in the imaging screen 1. These focus detection areas C detect the in-focus state regarding the subject A. These focus detection areas C correspond to a plurality of two-dimensionally arranged focus detecting pixels in an image pickup device having a plurality of two-dimensionally arranged pixels, for example, in the digital camera.
The tracking frame W is set in accordance with the size of the subject A. In the tracking frame W, the number of the focus detection areas C used to track the subject A varies by the size of the tracking frame W. For example, in the frame image shown in FIG. 21, the tracking frame W has a size corresponding to the size of the tracking frame W. For example, ten focus detection areas C are included in the tracking frame W in accordance with the size of the tracking frame W. In the frame image taken one frame later shown in FIG. 22, the tracking frame W tracks the subject A moving rightward, and moves in the same rightward direction to reach a tracking position B1. The size of the tracking frame W does not change with tracking, and the number of the focus detection areas C included in the tracking frame W remains ten.